Organic and inorganic nitrogen uptake by 21 dominant tree species in temperate and tropical forests

Abstract

Evidence shows that many tree species can take up organic nitrogen (N) in the form of free amino acids from soils, but few studies have been conducted to compare organic and inorganic N uptake patterns in temperate and tropical tree species in relation to mycorrhizal status and successional state. We labeled intact tree roots by brief 15N exposures using field hydroponic experiments in a temperate forest and a tropical forest in China. A total of 21 dominant tree species were investigated, 8 in the temperate forest and 13 in the tropical forest. All investigated tree species showed highest uptake rates for NH4+ (ammonium), followed by glycine and NO3- (nitrate). Uptake of NH4+ by temperate trees averaged 12.8 μg N g-1 dry weight (d.w.) root h-1, while those by tropical trees averaged 6.8 μg N g-1 d.w. root h-1. Glycine uptake rates averaged 3.1 μg N g-1 d.w. root h-1 for temperate trees and 2.4 μg N g-1 d.w. root h-1 for tropical trees. NO3- uptake was the lowest (averaging 0.8 μg N g-1 d.w. root h-1 for temperate trees and 1.2 μg N g-1 d.w. root h-1 for tropical trees). Uptake of NH4+ accounted for 76% of the total uptake of all three N forms in the temperate forest and 64% in the tropical forest. Temperate tree species had similar glycine uptake rates as tropical trees, with the contribution being slightly lower (20% in the temperate forest and 23% in the tropical forest). All tree species investigated in the temperate forest were ectomycorrhizal and all species but one in the tropical forest were arbuscular mycorrhizal (AM). Ectomycorrhizal trees showed significantly higher NH4+ and lower NO3- uptake rates than AM trees. Mycorrhizal colonization rates significantly affected uptake rates and contributions of NO3- or NH4+, but depended on forest types. We conclude that tree species in both temperate and tropical forests preferred to take up NH4+, with organic N as the second most important N source. These findings suggest that temperate and tropical forests demonstrate similar N uptake patterns although they differ in physiology of trees and soil biogeochemical processes.

Fate of atmospherically deposited NH4+ and NO3− in two temperate forests in China: temporal pattern and redistribution

“…Higher retention for NO 3 À is reported at sites receiving low ambient N deposition of 6-12 kg NÁha À1 Áyr À1 (Buchmann et al 1996, Providoli et al 2005, 2006. At sites that receive elevated N deposition in the temperate (Feng et al 2008) and tropical forests (Sheng et al 2014, Liu et al 2017, higher retention for NH 4 + input was evident. This is likely because NO 3 À is lost from these forests while NH 4 + can still be retained by abiotic mechanisms even under high N input (Preston and Mead 1994).…”

“…, Liu et al. ), higher retention for NH 4 + input was evident. This is likely because NO 3 − is lost from these forests while NH 4 + can still be retained by abiotic mechanisms even under high N input (Preston and Mead ).…”

“…, Liu et al. ). However, there is also evidence that conifers have strong ability to use NO 3 − (Rothstein and Cregg ).…”

“…For coniferous forests, preference for NH 4 + over NO 3 À was often reported (Kronzucker et al 1997, Wang and Macko 2011, Gruffman et al 2014) also because they naturally occur on NH 4 + -dominated soils, where NO 3 À concentration is virtually undetectable (due to low nitrification rates), thus they are viewed as strongly adapted to utilize NH 4 + (Lavoie et al 1992, Liu et al 2017. However, there is also evidence that conifers have strong ability to use NO 3 À (Rothstein and Cregg 2005).…”

Soil microbes promote complementarity effects among co‐existing trees through soil nitrogen partitioning

“…Unlike EM fungi that have access to organic N, AM fungi have no known saprotrophic capacity and function as an extension of roots to acquire inorganic N (but see Hodge, Campbell, & Fitter, 2001;Leigh, Hodge, & Fitter, 2009;Smith & Read, 2008;Smith & Smith, 2011). It has been shown that AM fungi can mobilize soil nitrate and transfer it to plant root cells (Azcón, Gomez, & Tobar, 1996;Azcón & Tobar, 1998;Liu et al, 2017). Thus, it is possible that tree species associated with either AM and EM fungi form complementary N-uptake niches that may promote their combined resource uptake, growth and productivity in forests.…”